Abstract by Mette Klitgaard Jensen

Oral drug delivery is the preferred route of drug administration. However, the development of new drug candidates is hampered by an increasing number of new chemical entities that exhibit poor solubility in water. The oral absorption of such drugs is limited by the solubility in the gastrointestinal tract. The drug development process includes in vitro characterization of the drug and drug delivery system followed by a preclinical study in animals such as rats. However, these in vitro studies often do not predict how the drug or drug delivery system behaves in vivo. Some suggest that the comparison fails as the in vitro studies are often performed simulating the human gastrointestinal tract rather than the gastrointestinal tract of the animal in which the in vivo study is conducted. Another theory is that the in vitro assessment of oral drug delivery systems often disregards the small intestinal mucus layer in the in vitro models, even though the oral drug formulation has to traverse the mucus layer in vivo to become absorbed in the small intestine after oral administration.

The current PhD project worked with the hypothesis that the mucus layer in the small intestine contains endogenous surfactants at levels that might benefit the oral absorption of poorly water-soluble drugs. To test this theory, the work of the project focused on firstly characterizing the conditions of the luminal fluids in different regions of the gastrointestinal tract of the rat relevant for the oral absorption of poorly water-soluble drugs; specifically the pH and concentrations of endogenous surfactants (bile salts, polar lipids, and neutral lipids). Secondly, the small intestinal mucus layer in the rat was characterized by focusing on the rheological properties, pH, and concentrations of proteins and endogenous surfactants. Lastly, the acquired knowledge of the rat small intestine was utilized to design media simulating the luminal fluids and mucus layer of four regions in the small intestine, i.e., the duodenum, proximal jejunum, mid jejunum, and ileum. The simulated media were then used to investigate whether the simulated regional differences of the small intestine affected the apparent solubility and permeation of the three model poorly water-soluble drugs, fenofibrate, griseofulvin, and dipyridamole.

In characterizing the properties of the luminal fluids, mainly two things became evident: Firstly, there are regional differences in the pH and concentrations of the endogenous surfactants in the gastrointestinal luminal fluids that would affect the absorption of poorly water-soluble drugs. For instance, the bile salt concentration increases until the mid-jejunum due to low bile salt reabsorption in this region and high water absorption. Secondly, the concentrations of endogenous surfactants were substantially higher than those previously reported in the human small intestine. This could explain why in vitro models using simulated human conditions to estimate oral drug absorption fail to predict the drug behavior in preclinical in vivo studies. 

The rheological properties of mucus displayed interregional differences in which the shear rate-dependent viscosity and storage and loss moduli of the two first sections were higher than the rest. The endogenous surfactants in the mucus layer of the small intestine were found to follow a similar trend of the regional differences as the luminal fluids. Compared to the luminal fluids, there were overall lower concentrations of bile salts and higher concentrations of fatty acids in the mucus. However, there was an overall higher concentration of surfactants in the mucus layer in the distal part of the small intestine than in the lumen, albeit the difference was not significant.

Eight media were designed to simulate the pH and concentrations of endogenous surfactants in the luminal fluids and mucus layer of the rat small intestine; i.e. rat simulated intestinal fluids (RaSIF) and rat simulated intestinal mucus (RaSIM) in the duodenum (D), proximal jejunum (PJ), mid jejunum (MJ), and ileum (I). RaSIM_D/PJ/MJ/I were further designed to reflect the rheological properties of the native mucus. The apparent solubility of the three drugs was all affected by the composition of the simulated media, especially the concentrations of endogenous surfactants. The permeation was evaluated in the mucus-phospholipid vesicle-based permeation assay. Only the flux of fenofibrate was observed to be affected by the interregional differences in the RaSIF_D/PJ/MJ/I and RaSIM_D/PJ/MJ/I, while the griseofulvin and dipyridamole flux across the barriers were unchanged between the different sections. The observed behavior of fenofibrate and griseofulvin seem to comply with previous studies of regional differences in oral absorption performed in rats and humans.

In conclusion, the work of this PhD thesis confirmed the presence of endogenous surfactants in the mucus layer of the small intestine at levels that might benefit the oral absorption of poorly water-soluble drugs. The PhD project had further expanded the understanding of the rat as a model for human oral absorption of poorly water-soluble drugs through the characterization of the gastrointestinal luminal fluids and small intestinal mucus layer of the rat. The work has led to the design of simulated media reflecting different regions of the rat small intestine, which can be used for species-specific in vitro models to improve the overall drug development process.